1. Field of the Invention
This invention relates to fluorocarbon coating compositions.
2. Prior Art
In recent years, the use of fluorocarbon polymers as lubricious coatings has become widespread. Cookware coated with fluorocarbon polymers, especially tetrafluoroethylene (PTFE), has become very popular; almost every housewife prefers to use such cookware in her kitchen because food is less likely to stick to it and because it is so easy to clean.
However, the fluorocarbon polymer's physical property which causes a lubricious coating also makes it difficult to bond fluorocarbon polymer coatings well enough to substrates to prevent the coatings from blistering and peeling away during use. This is especially true of PTFE.
This problem, involving the inherent nature of fluorocarbon polymers, is alleviated by first prime-coating the substrate with an adhesion promoting primer and then top-coating the primed substrate with a fluorocarbon polymer dispersion.
However, the known prime-coating compositions were inadequate for use with the newer, currently-used fluorocarbon polymer compositions which provide cookware top-coatings that are denser and less porous than the previous fluorocarbon polymer coatings. These current top-coatings make the cook-ware even easier to clean and improve their appearance by making the film glossier.
This inadequacy is because previously known prime-coating compositions contained a large quantity of water soluble salts, e.g., lithium salts. Water, which can penetrate the fluorocarbon top-coats, forms, in the presence of these water-soluble salts, high osmotic pressures which cause loss of adhesion and blistering. The previously used fluorocarbon top-coats had enough porosity to allow the salt solution to escape before the osmotic pressure buildup was sufficient to cause adhesion loss and blistering. The current, denser, less porous top-coats do not.
New prime-coat compositions were required and were developed to correct this problem. One example of such a new prime-coat composition is disclosed in Welch, Ser. No. 405,798, filed Oct. 12, 1973. These primers consist essentially of fluorocarbon abd colloidal silica. Although these compositions provide an excellent priming coat, they have a short storage life and require use during a relatively short time.
The coating composition of this invention provides an excellent priming coat and has a long storage life.